Joining pastes for purposes of the present invention are comprised of two components: a powdery joining metal composition in the range of about 50 to 90%; and a vehicle in the range of about 10 to 50%. The vehicle in turn consists of a solvent for fluidity, and second phase ingredients, including a fluxing agent and various special additives such as activators to increase the fluxing action, detergents, suspending agents or surfactants, thickeners, and defoamers. The particular composition is dependent upon the application contemplated.
Present-day precision joining applications involve highly sophisticated techniques that require very specialized properties of the joining pastes. For this purpose considerable effort has been expended in the art to develop specialized formulae, exacting material specifications, and careful storage and handling controls. However, little attention has been given to the development of improved processing techniques for the preparation of the pastes. Conventionally, paste compositions are simply prepared by blending all of the ingredients together at one time in a suitable mixing vessel. The mixing or blending may or may not be carried out at high temperatures. If done at high temperatures, cooling to storage temperature may simply be allowed to take place at ordinary cooling rate, dependent upon such factors as ambient temperature, size of batch and the vessel in which the cooling takes place.
One requirement of joining pastes suitable for precision soldering techniques, such as used in the manufacture of electronic components and jewelry, is that the paste be dispensible from fine bore dispensing equipment; for instance, dispensing equipment having a bore size of about 15 to 25 gage (about 0.05 to 0.01 inches). Even general automotive applications require relatively fine bore size equipment, for instance equipment with a bore size of 1/32 to 3/16 inches. It was found that techniques of manufacture conventionally employed resulted in the preparation of pastes which may be unsuitable for automatic dispensing in small bore equipment, either because of excessive particle size of paste ingredients, or because of paste instability. By paste instability, it is meant phase separation of ingredients in the paste (suspension stability), or increase in viscosity or hardening of the paste on storage (viscosity stability). Criteria for stability are that the paste be capable of storage, for at least six months, without visible phase separation, and with a viscosity change of less than about .+-.15% (in centipoises).